How to Blueprint Engines: Intake Manifolds Guide

The intake manifold provides a pathway for the intake air charge. On an engine that is carbureted or that has throttle body injection, the manifold carries the air and fuel mixture to the cylinder head. On an engine with direct-multi-port fuel injection, the intake manifold’s job is primarily responsible for ducting the intake air charge. Intake manifold design can have a huge impact on engine performance, affecting runner and port shape and volume.

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Manifold Types

Intake manifold design affects peak power and the RPM band where the engine produces maximum torque and power.

Following a hydrographics treatment, a high-performance clear-coat finish is placed on this intake. Even after a full day on an engine dyno and after spilling fuel during carb changes, the urethane clearcoat held up without a single blemish.

Single-Planes

A single-plane manifold (depending on overall height) can be produced with longer runners and the ability to improve alignment of the cylinder head’s intake port roofs. The larger plenum area with larger cross-sectional area provides a more direct shot at the cylinders, favoring the upper RPM band. The “straighter” runners tend to slow the mixture, which aids in reducing fuel separation (fuel droplets).

Single-plane manifolds are offered in a variety of runner lengths and overall heights. Generally, the longer the runners, the more suited the engine becomes for top-end power.

If the manifold has a divider plate, you can cut it down to increase plenum volume for better top-end performance. Rather than messing with the divided plate, try installing a carb spacer (experiment with thicknesses).

Single-plane intake manifolds are generally designed to allow optimum airflow at higher engine speeds. Single-plane manifolds have a larger plenum volume area, and the runners flow more directly at the cylinder head intake ports.

Dual-Planes

A dual-plane manifold has a split plenum with an upper plenum pocket directing the charge to four select cylinders. The lower plenum pocket directs the charge to the remaining four cylinders (delivering air/fuel to cylinders firing 180-degrees apart).

The runners are generally longer (better for low-end engine torque) and of a smaller cross-sectional area (which increases velocity).

A dual-plane intake manifold is usually the best choice for street driving because the design is targeted at idle, low-end torque, and low-end throttle response.

Some tunnel ram intake manifolds, such as this one from Pro-Filer, have a modular plenum top. This not only gives you a choice between using a single or dual carb setup, but the open access makes it very easy to inspect for port matching to the cylinder heads.

The plenum top attaches with a series of bolts and is sealed with a one-piece gasket.

A dual-plane manifold with a divider wall. Each half of the plenum directs the fuel/ air flow to half of the cylinders. Note the grid pattern in the cast floor of the plenum. This is intended to help bust up the mixture for better atomization.

Intake manufacturers have devoted years of development and are always searching for ways to improve airflow and to maximize engine performance at specific ranges of engine speed. One example is Edelbrock’s Performer RPM AirGap dual-plane manifold (developed from the Performer RPM). The runners are more isolated from engine heat and the height was increased for additional plenum volume. This improves the compromise between a dual-plane and singleplane, maintaining low-end performance of a dual-plane while increasing top-end power.

While the aftermarket offers dualplanes that focus on low-end idle quality, single-planes focus on the top end. Either design is slightly altered to broaden their ranges, and therefore multiple choices abound. I wish I had the space to devote to this subject in depth here, but suffice it to say that hood clearance issues aside, a dual-plane is likely the best choice for street cruising, and a single-plane is likely the best choice for high-RPM performance.

Tunnel Rams

A tunnel ram intake manifold is simply a larger, longer, taller version of a single-plane manifold. Although a tunnel ram system really isn’t a good choice for ordinary street driving, everybody thinks that a tunnel ram with dual carbs just looks cooler and they convince themselves that they can’t live without this setup.

A tunnel ram can be suitable with careful tuning and component selection. A pair of 650-cfm carbs is a usual choice, but the tunnel ram manifold needs to A dual-plane intake manifold is usually the best choice for street driving because the design is targeted at idle, low-end torque, and low-end throttle response.

A cross-ram intake manifold is another type of single-plane design, but with even longer runners than those on a tunnel ram.

Wet versus Dry Manifolds

One very basic difference between a carbureted intake manifold and a manifold used for fuel injection deals with the nature of flow. A carbureted intake manifold is referred to as a wet manifold because a mixture of air and fuel is fed through the manifold on its way to the cylinder head. A manifold for fuel injection is referred to as a dry manifold, since only air is flowed through the manifold (with injectors introducing fuel spray at the exit area of the manifold runners).

If the manifold is designed for a tall-deck aftermarket block, there may be a gap at the front and rear manifold rails. In this case, a manifold rail spacer kit is available to fill this gap.

Because of this basic difference, the surface finish and shape requirements of the intake manifold runners can differ. Although many variables come into play in the specific build, the surface finish may be more important in a wet manifold. This is because you’re dealing with flow, tumbling, and velocity of a wet (air and fuel) mixture (with the surface finish playing a role in how the fuel droplets can cling or atomize during flow). With an injected manifold, surface finish is likely less critical due to the absence of fuel handling.

Port Matching

Port matching refers to achieving a proper alignment and shape between the intake manifold intake ports and the cylinder head intake ports. Depending on the cylinder head and manifold selection, a slight mismatch can occur, resulting in an airflow interruption or turbulence as the charge leaves the manifold runner and enters the cylinder head port. Most commonly, the intake manifold port exit is then modified in order to match the exact location and shape of the head intake port. This usually involves grinding material from the manifold runner port.

The rail spacers are predrilled for use with locating pins. Each rail is machined to conform to the front and rear block rails.

Installing locating pins ensures that the spacer installs at the correct location, and that the spacer can’t accidentally move during assembly or during engine operation.

Verify that the installed rail spacer matches the block deck angle.

A neat way to seal is with RTV. Before applying RTV to block/manifold end rails, carefully mask off the block face and manifold rail face (or in this example, the rail spacer). Apply the RTV, allow a few minutes for partial setup, and install the manifold or spacer. The excess RTV can be wiped off without smearing the block or manifold. Once the excess is removed, carefully remove the masking tape. The result is a sealed joint that looks neat and tidy.

The first step in port matching is to carefully measure the cylinder head intake ports and the intake manifold ports. If the intake manifold ports are wider (for example) than the intake ports on the head, first grind to widen the head ports to match the same width. Then grind the intake manifold roof and floor areas to match the head. The goal is to have the same port size at manifold and head, and to have them align with no steps or interruptions. However, it’s common to size the intake manifold ports just a bit smaller (by about .015 inch or so) to accommodate any play or slop in the manifold bolt holes.

Before you start grinding, establish the fixed reference points; use the block that you intend to use with deck height finished. Install the heads with the exact type of head gaskets to be used during final assembly, or shim the heads to mimic the head gasket thickness. Also shim the space between the manifold and heads with the same thickness of the crushed intake gaskets (it’s best not to use actual intake gaskets because they may interfere with precise outline scribing).

Place the manifold onto the block and heads and tap it down to make sure it’s fully seated. Check to see if either the floor or roof of the ports aligns with the head ports. It’s best to establish roof alignment. You can change shims to raise or lower the manifold to achieve roof alignment (just remember to use intake gaskets of the same thickness during final assembly). Slide the manifold fore and aft to check for common-wall alignment (the thin walls that separate ports).

Apply machinist’s dye to the manifold deck around each port. Using calipers, measure the head ports (height and width). Using the previously established roof height as the index, use a precision straightedge and scribe a horizontal line across the entire manifold deck, in-line with the roofs. Using the height and width measurements taken from the cylinder head ports, use the straightedge and a scribe to mark the horizontal (floor) and vertical (wall) guides onto the manifold decks (you’re simply transferring the head port locations to the manifold).

Select a cutter bit with the same radius as the port corner radii (if in doubt, apply machinist’s dye to the head port corners and hand roll the cutter in the corner to see if the cutter makes full contact with the corner radius). Using a radius-nosed cutter bit on a controllable-speed electric die grinder (more controllable than pneumatic), begin to cut the ports edges exactly to the scribed outlines. Once each edge is cut open to the scribe line (and straight), blend the grinding into the port at a depth of about 1 inch (possibly shorter, depending on the manifold design). Do not grind beyond your scribe marks. Remember, for aluminum cutting, you must use a wide-flute bit designed for aluminum (otherwise you clog the flutes).

Once all ports have been cut, finish by smoothing and polishing with a 60-grit abrasive roll on the die-grinder tool. If you want a smoother finish, follow this with an 80-grit roll.

You can also take advantage of intake manifold gaskets in your attempt to port match. For instance, if the floor of the manifold ports are a bit lower than the floor of the head intake ports, you could move to a thicker intake gasket to slightly raise the manifold. For popular engines, manifold gaskets are available in a variety of thicknesses. Consider this before you start hacking away at the manifold or head.

If using different intake gasket thicknesses doesn’t solve the initial alignment (for manifold installed height), it may be necessary to remove material from the intake manifold mounting flanges to achieve good port alignment and sealing surfaces.

Port Matching and Machining

The following are some general rules for decking and milling blocks, heads, and intakes for port alignment:

If a cylinder head is angle milled, the intake face on the cylinder head must be adjusted at the same angle, with minimal material removed. It then may be necessary to increase the diameter or create slotted bolt holes in the intake manifold for attachment to the cylinder heads.

For 90-degree V-8 engine blocks, for each .010 inch removed from the head or block deck, the intake port opening is expanded by .007 inch. In order to bring the port alignment back into position, the intake manifold needs .005 inch of material removed from each side to effectively make the manifold seat lower on the heads.

In most instances you can just divide the total amount of decking on one bank in half to calculate intake manifold adjustment.

Port matching really isn’t necessary unless you’re trying to optimize engine performance or encounter serious misalignment for tolerance stack-up. If you’re simply toodling along on your way to a show or the local malt shop, don’t fret over this.

Intake Plenum

For a carbureted manifold (or a carb-style manifold that uses a throttle body), take a look at the plenum divider walls. Remove any imperfections (casting bumps, flashings, etc.). In order to aid in airflow or fuel/air flow, address the dividers. The dividers typically (not always) have rough or almost squared off edges. Using a grinder or abrasive roll, radius these edges to a bull-nose shape (not to a sharp knife edge). The goal is to remove sharp edges and/or abrupt surfaces that could create excess boundary layers (turbulence).

Before you start checking port alignment, apply machinists’ dye to each port opening. This allows you to easily see any scribe lines.

After using the port roofs as the reference points, and after measuring the cylinder head ports, the dimensions are transferred to the intake manifold.

In some cases, where the manifold ports are smaller than the head’s ports, using a U-bent wire allows you to reach into the manifold, using the short vertical section to guide you around the head port, while scribing outlines around the manifold ports. Here Jim, of Fox Lake Racing Heads, prepares to scribe a lower intake manifold for a 5.0L injected Ford.

A machinists’ precision straightedge is used to scribe common floor lines across the entire bank of ports.

When cutting aluminum, be sure to use a cutter with large flutes designed for aluminum.

After removing material around the port edges to your scribe lines, taper blend the cut inside the runner at a depth of about 1 inch.

Blend into the port to fully smooth out the surface, again, at about a 1-inch depth. If you prefer a smoother surface (more of a polish), then go over the surfaces with 80-grit.

Depending on engine design/type, the manifold may have port pairs that are placed very close together. When port matching, align these divider walls to the heads first, and be careful not to eliminate the gasket sealing area.

The popular F.A.S.T. composite intake manifold for fuel-injected applications consists of a lower section (runners) and a top section that incorporates the throttle body inlet. Shown here is a unit for late-model Corvette.

Plastic intake manifolds for fuel-injection setups (OEM or aftermarket) are basically bolt-ons. Due to the weight-saving designs and preregistered elastomeric-seal grooves, you really can’t modify the ports.

Round off the edges of the port dividers to achieve a radiused profile. There’s no need to knife-edge these walls.

If you plan to use a carburetor spacer (to increase plenum volume for better top-end performance), apply machinist’s dye to the manifold’s carb mounting pad and install the spacer. If any manifold pad material extends beyond the inside walls of the spacer, scribe a line using the inside of the spacer as a template. Remove any exposed material from the carb pad to remove any obstructions to flow (match the manifold’s plenum opening to the spacer). Gently blend this area of the carb into the plenum opening.

Intake Port Surface

Although a fully polished surface that looks chrome plated may look really cool, this is usually not necessary. Finishing with an 80-grit abrasive roll is adequate polishing. Polishing is more important for any sharp turns in the flow path, which is where flow speeds are the highest. The shorter the turn, the more need for polishing.

Also, the need (or benefit from) polishing can vary depending in part on the size of the manifold runners. Smaller runners can benefit more from surface polishing than larger port runners. Smal-lvolume runners may be more sensitive to turbulence factors that can result from casting surface boundary layers.

Manifold Bolts

It’s common for a novice to undertighten, overtighten, or unevenly tighten intake manifold bolts. Any of these can result in vacuum, oil and/or coolant leaks, and a warped or cracked manifold.

Always follow the bolt torque specs and the specific tightening sequence recommended by the intake manifold manufacturer. A fairly common challenge, simply due to the design of the manifold, is gaining adequate access to certain manifold bolt locations. In some cases it may be difficult to access the bolt heads with a straight socket wrench.

It is common for installers to use an open-end wrench and to guess at torque value. In locations were you cannot access a bolt directly with a socket wrench, the answer is to obtain an offset wrench extension. This places the wrench (at the bolt head) away from the centerline of the tool’s drive head (making the effective overall length of the torque wrench tool longer or shorter).

When using an offset wrench, you must make a compensation adjustment in order to achieve the desired torque value. Otherwise, you unknowingly overtighten or undertighten the bolt. If the extension points out and away (but in-line with the torque wrench body) from the torque wrench drive, this obviously lengthens the overall tool length. If the extension is installed to the torque wrench 180-degrees (in-line with the torque wrench body but now underneath the tool), the effective length is shorter.

Use this formula so the adapter makes the wrench longer:

TW = (L ÷ L + E) x desired TE

Use this formula so the adapter makes the wrench shorter:

TW = (L ÷ L – E) x desired TE

Where:

TW = Torque setting on the torque wrench

L = Lever length of the torque wrench itself (from center of the wrench drive to the center of the torque wrench hand-grip area)

E = Effective length of extension, from the center of its square drive hole to the center of its wrench head

TE = Torque applied by the extension to the fastener

If you want to know where to set the torque wrench when using an adapter that alters the effective length of the wrench, you must calculate to compensate for the adapter. If the distance from the wrench drive to the center of the bolt makes the wrench longer, the final wrench setting must be adjusted to a lower value in order to compensate.

As an example, an intake manifold bolt torque value is listed as 30 ft-lbs. In order to access a hard-to-reach bolt, you may need to use a 2-inch offset extension wrench. In this case, the torque wrench measures 12 inches from the center of the drive to the center of the wrench handle. With the wrench extension pointing away from the wrench drive, this changes the distance from the center of the bolt to the center of the torque wrench grip to 14 inches (making the torque wrench 2 inches longer).

For this example, the formula works out like this:

TW = (L ÷ L + E) x desired TE

12 ÷ (12 + 2) x 30

12 ÷ 14 x 30

.9 x 30

27

In this example, the wrench is set at 27 ft-lbs, in order to actually achieve 30 ft-lbs.

If the wrench extension is aimed toward the handle (turned 180-degrees from the prior example), and you still want to achieve 30 ft-lbs of torque, you know that the adapter has now made the wrench shorter (because the center of the bolt is now closer to the center of the wrench handle).

Example of a dual-plane manifold for the Ford FE engine. Note the pushrod holes.

Wrench extensions are readily available from quality tool sources. Just remember to adjust the torque wrench accordingly to compensate for the added leverage.

The vintage Ford FE big-block manifolds have pushrod holes. For high-performance applications where larger diameter pushrods and/or high-lift cams are used, these holes may require slight enlargement or elongation. This must be checked during test fitting, with an assembled short block, heads, and rockers installed.

For those situations where you simply don’t have access with a socket wrench, a wrench extension makes it easy to apply the desired torque value to an intake manifold bolt, rather than just guessing at bolt tightness.

The MSD 6LS ignition module and harness makes converting an LS engine to carburetor extremely easy. There are two versions: the 6LS (PN 6010), designed for use with the 24-tooth crankshaft reluctor wheel (earlier LS1/LS6) and the 6LS-2 (PN 6012) for the later 58-tooth reluctor wheel (LS2 and others). Just make sure that you purchase the correct controller based on your reluctor wheel tooth count.

Converting a GM LS engine to old-school carburetion is embarrassingly simple, requiring only a manifold, carb, and an MSD ignition controller. No ECM required.

For this example, the formula works out like this:

TW = (L ÷ L + E) x desired TE

12 ÷ (12 – 2) x 30

12 ÷ 10 x 30

1.2 x 30

36

In this example, the wrench is set at 36 ft-lbs, in order to actually achieve 30 ft-lbs.

If the adapter makes the torque wrench longer, you must reduce the setting on the torque wrench. If the adapter makes the torque wrench shorter, you must increase the setting on the torque wrench.

Converting LS to Carb

The Gen-3 and -4 GM LS engine was originally designed to feature electronic multi-port fuel injection. For those who prefer to run a carburetor, for street rod, custom car, or racing, the swap is relatively easy. The only components required include a carburetor, intake manifold, and ignition controller system. No on-board computer is needed. You retain the engine’s ignition coils, crankshaft position sensor, camshaft position sensor, water temperature sensor, and manifold absolute pressure (MAP) sensor.

Because the carb and manifold handles fuel/air delivery, no electronics are involved. You simply need a way to time ignition. The commonly used controller for this application (and the only one I’m aware of) is the MSD 6LS controller, which includes a selection of six plug-in chips, each with its own ignition curve. Refer to the MSD instructions, pick the curve you want to try, plug it in, and go play. It’s that simple. If you prefer to map your own curve, a CD is included that lets you program the curve on your PC.

Be aware that you need to buy the correct controller for the tooth count on your crank’s reluctor wheel. The MSD 6LS (PN 6010) is compatible with a 24-tooth wheel (LS1/LS6 and early LS2), and the 6LS-2 (PN 6012) is for the later 58-tooth wheel (later LS2, LS7, LS3, and LS9).

Manifold Surface Protections

Okay, so you purchased a hot-dog aluminum intake that’s gonna boost the pony count, and it looks way cool squatting between those trick aluminum heads. However, a few months down the road, the manifold starts looking funky. It’s either scuzzing up with a white-hued film that rubs off like chalk or it starts looking brownish, as though it were rusting. How can this be? After all, it’s aluminum.

If the manifold begins to turn brown (as though a light surface rust were occurring), you’re not imagining this. If the aluminum manifold was steel-shot blasted during the manufacturing process, particles of steel may have been imbedded in the aluminum surface. Exposed to the elements, the steel begins to oxidize (if clean stainless steel shot is used, this does not happen).

If the aluminum surface begins to oxidize on its own (a result of moisture due to humidity or being exposed to water in some other manner), a white film begins to develop. Left unattended, this can lead to long-term pitting.

Penetrating Lube

In order to keep the new manifold looking, well … new, you have a few pre-installation options for treating the surface to prevent oxidation.

The cheap (and somewhat effective) route is to carefully wipe the exterior surface with a soft steel wool pad (or red Scotchbrite pad) that has been soaked with a penetrating lubricant such as WD-40. Using a bit of effort, rub the lube into the entire outer surface, including every nook and cranny. This changes the appearance, providing a slight burnished look and ever-so-slightly darkening the aluminum, yet providing a still-attractive appearance.

Once you’ve applied the penetrating lube into the surface with the aid of a light abrasive pad, rinse the manifold thoroughly with hot water to remove all abrasive particles. Dry the surface and immediately re-apply the thin penetrating oil using a soft clean rag. This generally keeps the intake looking spiffy for a season or two, and helps to prevent future staining. Down the road, you can always clean the installed manifold surfaces and re-apply the lubricant with a clean rag. When the light oil is applied, don’t leave it wet; dry and buff with a soft, clean rag (otherwise, the oil attracts dust/dirt particles). This may seem to be an archaic method, but it works as long as you maintain it via routine wipe-downs (detail cleaning).

Cleaning a bare (as-cast) intake manifold can be tricky if stained by fuel or road and weather elements because of the porous nature of an as cast surface. Commercially available aluminum cleaners include OxiSolv and Evapo-Rust. It’s important to follow the instructions included with these cleaning products.

Tumble Treatment

Another option is to have the manifold tumble-treated (where the manifold is gently tumbled in an immersion of small polishing stones). Depending on the size, shape, and composition of the media, this burnishing process smooths the surface to a satin, semi-polished, or full-polished finish, depending on what result you want. Although this doesn’t provide a protective coating, it diminishes or eliminates casting surface texture, which makes it much easier to clean. Of course, this can always be followed up by the application of a protective film or coating.

Coatings

You can also have the manifold professionally treated or coated with a protective finish. This can be done by adding a Teflon coating (the surface darkens but prevents moisture and other deposits from sticking to the surface), a ceramic coating (which generally brightens the surface, depending on the formulation), or a powder coating.

A good powder-coating shop can provide virtually any finish you want, including clear, a color , a smooth finish, a wrinkly finish, pebble finish, etc. There are plenty of good powder coating shops around.

A race manifold (usually) shouldn’t have a barrier coating (for heat dispersion), but for a street/show rod, appearance is paramount, so do what you need to do in order to maintain the beauty factor. Discoloration, stains, or oxidation can occur from weather conditions (humidity, airborne pollutants), fuel leaks, coolant leaks, etc. The proper protective coating eliminates surface oxidation and allows easy cleaning of other surface contaminants.

If a performance coating is desired, check with the major engine-component coating services. They offer a wide range of specialty coatings designed for whatever goal you have in mind, including anti-friction, heat barrier, heat dispersion, faster oil drainback, etc. These sources include (but are not limited to) Swain Tech Coatings, Polydyne, Calico Coatings, and TechLine Coatings.

Concerning intake manifolds specifically, underside coatings (where the bottom of the manifold faces the lifter valley) are available that provide a heat barrier (keeping the manifold cooler) and coatings that prevent oil from clinging (for quicker oil drainback).

Some intake manifold manufacturers offer their manifolds already treated with some type of protective coating as standard or as an option. If yours is delivered bare, seriously consider applying some type of surface protection simply to maintain a like-new appearance.

Hydrographics (wet ink film graphics) provides a simple and relatively inexpensive way to create a unique visual impact. This manifold was ground smooth, polished, then delivered to a dipping service. The techs at Dip ’N Designs (Wooster, Ohio) applied a black basecoat, carefully dipped the manifold through a water surface suspended with carbon-fiber ink film, rinsed, dried, and then clear coated the manifold. Since the graphic ink film is somewhat translucent, the black base coat was needed to achieve the desired hue.

Overhead view of the ”carbon fiber” manifold on a Pontiac 455 that we bored/stroked to 501 ci.

Topped off with a dual-snorkel vintagestyle air cleaner, the intake/carb swap from its previous fuel injected format resulted in taking advantage of currentday LS long-block technology while retaining a vintage performance look.

Using the Edelbrock dual-plane intake and a 650-cfm Holley (and a mild-cam change), we simply overbored this 5.3-liter LS iron-block to 327 ci and hit 450 hp. The manifold provided snappy throttle response and favored bottom-end and mid-range power.

Topped off with a dual-snorkel vintagestyle air cleaner, the intake/carb swap from its previous fuel injected format resulted in taking advantage of currentday LS long-block technology while retaining a vintage performance look.

Actual chrome plating, while attractive, probably isn’t a wise choice, simply because the plating process (copper, nickel, chromium) can trap heat within the manifold, more so than other treatments. If you want a chrome-like finish, a good powdercoater can achieve this for you. The coating shop may also be able to apply a chrome-type finish in other colors than nickel-chrome.

Hydrographics

This is also called wet-dip. It involves a preprinted ink film (graphics of your choice, such as carbon fiber, camouflage, etc.). The film is laid onto the surface of water in a temperature-controlled tank. The component (in this case a manifold) is carefully lowered onto and through the ink film, much like dipping an Easter egg. The film adheres to the manifold surfaces, wrapping all contours. The manifold is then removed, rinsed, dried, and treated to a protective urethane clearcoat. For best results, the manifold’s exterior surfaces need to be fully smoothed and polished prior to dipping.

Faux Fiber

I recently had a 4-barrel manifold (for a 501-ci Pontiac engine build) treated to a carbon-fiber graphic. I spent a few hours deburring the intake (removing casting flashings, casting-mold lines, etc.) and fully polishing the entire outer surface. I delivered the manifold to Dip ’N Designs. Because I wanted a black carbon-fiber appearance, they first applied a black basecoat (the film is somewhat translucent, so the undercoat influences the final hue), followed by dipping, rinsing, drying, and clearcoating. The result was spectacular. At a major performance trade show, everyone who inspected the Pontiac engine thought that the intake manifold was actually made of carbon fiber.

This graphic treatment does hold up to engine heat and contact with fuel. We ran this engine on a dyno for a full day, with no visible effects from the heat (no discoloration, no cracking, no lifting). Even with fuel spilled onto the surface during carb changes, the urethane clearcoat seemed impervious.

The only tip that I’d like to pass along deals with the intake manifold bolt-hole locations. The clamping force of the intake manifold bolt heads and washers tend to compress and raise the clearcoat around the edges of the washers. To avoid this, lightly spotface each bolt hole (flat-spot-face each bolt hole to slightly exceed the outer diameter of the washer). The spot-face doesn’t need to be very deep, just enough to register the washer. The spotface should be a few thousandths of an inch larger in diameter than the washer (for example, if the washer OD is .450 inch, the spotfacing should be about .470 inch). After ink-dipping and drying, the graphics technician can then carefully mask each spotfaced recess before applying the clearcoat.

Carb Spacers

Although some builders take advantage of a carb spacer simply to provide needed clearance between the carb’s fuel log and intake manifold (in cases where fit poses a problem), spacers are normally used to help tune the engine’s performance band. Spacers don’t add power; rather, a spacer can be used to tune the power band in the RPM range can only use generalities about each style of spacer, since each specific engine application has its own set of variables (a setup that works well on one engine might not act the same on a different engine).

Designs

Spacers are available in different designs, including four-hole, single-hole, open, combination, and with a plenum divider. There’s more to choosing a spacer than simply basing the selection decision on thickness.

A four-hole spacer (four holes that align with the carburetor barrels) generally increases throttle response and acceleration, and generally increases torque by moving the power band to a lower RPM range. The four-hole design forces the column of air moving from the carb into the intake plenum to take a longer path (flowing longer), which increases air velocity.

A spacer with a single, large, opening tends to raise the power band to a higher RPM range (less bottom end but more top end). This type of spacer also has a center divider plate that splits the plenum path left-to-right. For applications where rich/lean conditions exist on the engine’s left and right sides during turns, the divider plate helps to equalize fuel/air distribution for a more even feed to all cylinders. This is generally not required for street application, and is more targeted to oval track or some road-race applications. More advanced spacers are also available with tapered holes, where the taper angle has been developed for optimum performance on specific manifold/carb/cam setups.

An open spacer increases plenum volume by increasing the distance between the carb and plenum floor. The open type generally decreases throttle response.

A combination spacer has a fourhole design along with a relieved floor area (basically, a combination of a fourhole and an open style). The combination spacer design generally helps to increase throttle response, while also widening the torque and power band throughout the RPM range (sort of the best of both worlds). The top of the spacer (mated to the carb) has the four holes flush to the carb, while the underside of the spacer is relieved in a square pattern (encompassing the entire group of the four holes) to slightly increase plenum volume.

Materials

Spacers are offered in a variety of materials, including wood, plastic, phenolic, and aluminum. Stay away from wood and plastic (wood is a good heat insulator but can absorb fuel, and plastic isn’t very strong and can crack).

Phenolic fiber is a good heat insulator and is a good choice. But if you plan to modify the spacer (custom porting), phenolic isn’t very workable.

Aluminum (cast or billet) isn’t the best heat insulator, but it’s easily modified and, in the case of today’s precision-machined billet and color-anodized choices, is a great choice, especially for a custom application where appearance is key (a sexy red or blue anodized spacer can add some real pizzazz).

Thickness

Aside from providing mounting clearance, spacer thickness is a tuning variable. The thicker the spacer, the more you increase plenum volume.

For the typical owner who doesn’t have access to a dyno or flow bench, determining the optimum spacer thickness for a specific application involves some trial and error. Luckily, swapping spacers is easy. Remember, the shorter the spacer, the more low-end torque and power available. The thicker the spacer, the torque and power band moves to a higher RPM range.

If you do plan to play with spacer thicknesses, start with carburetor mounting studs that accommodate the thickest spacer that you have in mind. This eliminates the need to install specific-length studs during each spacer swap.

Written by Mike Mavrigian and Posted with Permission of CarTechBooks

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